Steel mill tackles power quality and power factor problems

North Star Steel confronted low power factor (PF) and harmonics problems prior to installing a new ladle furnace by conducting a computerized analytical study using harmonic and power factor software. This study included field measurements of harmonics and PF on various feeders in the system. The Beaumont, Tex. company determined the characteristics of existing equipment and conductors. The objective

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North Star Steel confronted low power factor (PF) and harmonics problems prior to installing a new ladle furnace by conducting a computerized analytical study using harmonic and power factor software. This study included field measurements of harmonics and PF on various feeders in the system. The Beaumont, Tex. company determined the characteristics of existing equipment and conductors. The objective was to bring the overall plant PF to a value above 0.90 lagging, which would eliminate utility penalties.

At the time of the study, the plant had two existing scrap metal furnaces, and the electrical system included corrective PF capacitors as components of existing harmonic filter banks. A rolling mill connected to a 13.8 kV system having no PF correction was also in operation. (See Fig. 1, on page 49, for a simplified one-line diagram.)

[Figure 1 ILLUSTRATION OMITTED]

Developing computer simulation models

A steady-state harmonic model of the plant was developed based on obtained information on existing loads, conductors, and field measurements. Testing was done at various locations within the facility to determine harmonic voltages and currents resulting from existing scrap metal furnace and rolling mill operation. Measurements at the primaries of the furnace transformers, the main utility entrance busses, the existing harmonic filter banks, the main feeders serving the 13.8kV bus, and the primary feeders serving the rolling mill busses were made. Testing was carried out during concurrent operation of the scrap metal furnaces (arc load) and the rolling mill (drive load). To perform the harmonic analysis, including the future ladle furnace, typical data obtained from harmonic field measurements of similar ladle furnace installations were used for simulation purposes.

Harmonic field measurements for 13.8kV systems

Power factor correction requirements at the steel mill were determined through field measurements via testing and computer analysis with the help of power factor and harmonic software. Using portable harmonic monitoring instruments, on-site measurements were performed at the main switchgear feeder circuit breaker locations on the 13.8kV system for the rolling mill loads over several days. Existing current and voltage transformers were used as signal sources at all measurement points. Power flow and power factor data also were gathered to aid in the system analysis.

The voltage THD measurement was in the range of 1.51% to 8.17% of the fundament voltage. The voltage range is due to the load level of the mill stands (a machine that "mills" or rolls the steel into various shapes or different thickness). When the mill stands are working hard, there is high current flow and high THD. When the machines are idling between loads, the result is low current flow and low THD. The current THD was in the range of 3.45% to 9.86% of the fundament current. The following total power flows into the 13.8kV bus were measured at the typical heavy loading periods.

[MW.sub.measured] = 23.3MW

[Mvar.sub.measured] =19.9Mvar

[MVA.sub.measured] = 30.6MVA

[PF.sub.measured] = 0.76 lagging

The mill stand motor drives are the largest harmonic producing load on the plant's electrical system. Connected through step-down transformers, these drives are 6-pulse units generating predominantly a 5th harmonic current during operation.

From the standpoint of the power supply, the rolling mill is divided into two sections. (See Fig. 1.) The section served by Supply Circuit No.1 contains most of the 6pulse drives. The current THD on this supply circuit ranged from 27.28% to 34.16% of the fundamental current. [See Fig.2 (on page 50) for a typical current waveform for the 6-pulse drives.] This waveform contains 5th, 7th, 11th, and 13th harmonics. The current THD measurements taken on Supply Circuit No. 2 ranged from 3.69% to 7.76% of the fundamental current.

[Figure 2 ILLUSTRATION OMITTED]

Harmonic field measurements for 34.5 kV system

For this system, the voltage THD was normally in the range of 0.53% to 6.3% of the fundamental voltage. During times of erratic furnace arcing conditions, voltage THD values escalated, with a high of 19.18% being recorded on test instruments. The current THD on the two main feeders were in the range of 3.14% to 16.08% of the fundamental current. When both scrap metal furnaces were in operation, higher current THD was reported. The scrap metal furnaces are a low PF load, typically in the range of 0.65 to 0.85 lagging. With two harmonic filter banks already operating, the overall PF was in the range of 0.90 lagging to 0.90 leading.

Power factor study

Analysis of the power measurements taken at the site using test instruments indicated that reactive compensation was needed at the main 13.8kV bus to correct the PF from an average value of 0.76 lagging to 0.95 lagging. This reflected an initial low PF for the plant. The study also determined that existing harmonic filter banks on the 34.5kV system would provide adequate compensation for the additional reactive load of the new ladle furnace. This is because the 34.5kV system, at times, operated at 0.90 leading PF. Thus, the existing capacitors of the harmonic filters had the capability to correct the low PF from the new load as well as serve the existing load.

Field measurements using high-grade test instruments indicated the 13.8KV system load was approximately 23.3 MW with a 0.76 lagging PF, with an uncorrected reactive load of 19.9Mvar. To improve the 13.8kV system PF to 0.95 lagging, the reactive load had to be reduced to 7.2Mvar. An additional capacitor bank of 12.7Mar would provide the reactive compensation.

Analysis shows need of harmonic filters

A harmonic computer program was used to perform a steady-state analysis of the facility's electrical system for each frequency at which a harmonic source was present. The program calculates the harmonic voltages and currents throughout the system. In the harmonic simulations, the PF correction capacitors were connected to check for system resonance. The utility impedances were varied to simulate changes to the short circuit MVA.

A harmonic computer simulation of the 13.8kV power system, with 12.7Mvar reactive compensation added to the main bus, was made and revealed a parallel resonant peak close to the 4.7th harmonic, as shown in Fig. 3.

[Figure 3 ILLUSTRATION OMITTED]

A computerized harmonic analysis was done to determine the effect of the 12.7Mvar capacitor bank on the 13.8kV system. The calculated voltage THD more than tripled compared with the measured values, and the current THD reached values as high as 62% due to the parallel resonant peak. Corrective action had to be taken.

The parallel resonant condition on the 13.8kV system was determined by harmonic computer simulation. The peak near the 5th harmonic would cause harmonic amplification and indicated the need for a filter at this frequency. A 4.7th harmonic was selected for the tuned frequency filter to allow for inexact tolerances in the filter components and to reduce filter duty. The following equations were used to calculate the correct size of an in-line reactor to convert the 12.7Mvar capacitor bank into a 4.7th harmonic filter:

Calculating an [X.sub.C] of 14.995 ohm for a 12.7Mvar capacitor bank applied at 13.8kV results in a required [X.sub.L] of 0.679 ohm for a 4.7th harmonic filter.

An electrical model of the plant's distribution system with a capacitor bank installed was made, and an analysis was carried out to determine various electrical parameters of the steel mill's electrical system; the system did not perform as desired. Then, while modeling the electrical system, the capacitor bank was changed into a 4.7 harmonic filter bank by adding reactors and resistors. Results showed a decrease in voltage and current THD from the values obtained during the testing of the electrical system prior to the installation of the filter bank. Maximum voltage THD was 2.63%, below the 5% limit recommended by EKE, and maximum current THD was 3.83%, well below previously measured values and below recommended EKE limits.

Capacitors used with reactors as part of a filter will experience a steady-state voltage above the nominal line-to-neutral voltage of the electrical system. The excess voltage is a function of the harmonic number at which the filter is tuned. When installing capacitors in a harmonic filter, make sure the voltages and currents are within the following limits established by EKE/ANSI standards regarding capacitor nameplate rating: rms voltage less than 110%; peak voltage less than 120%; rms current less than 180%; and reactive power output limited to 135%. Harmonic analysis verified that the highest values of [V.sub.rms], [V.sub.peak] and [I.sub.rms] across and through the filter bank capacitors were all below these standard ratings.

A facility's electrical operation often can be improved by obtaining information on the harmonics and the power factor being experienced using test instruments and by taking corrective measures based on studies, via computer analysis. The studies will determine what will be needed to reduce voltage THD on the system, reduce current THD injected into the utility system, increase bus voltage and improve power factor and eliminate power factor penalties. For the steel mill, the computer model created for the harmonic analysis included data to anticipate the effect of the new ladle furnace installation. The filter bank for harmonic suppression and PF correction was designed to handle future loads as well as correct existing problems. Since nonlinear loads are typical in industrial facilities today, properly specified and installed harmonic suppression filters are an effective way to reduce harmonics and improve plant power factor.

Beware of harmonics

When loads are linear (such as conventional induction motors), the voltage and current are essentially sine waves, and a form of power factor called displacement power factor (DPF) is present. The ratio of useful working current to total current in the line, or the ratio of real power to apparent power, equals DPF. The use of certain AC equipment, primarily induction motors, requires power lines to carry more current than is actually needed to do a specific job. If properly applied, capacitors will supply a sufficient amount of leading current to cancel out the lagging current required by the inductive load. Capacitors thus reduce the total current flowing through the electrical system, improving system efficiency.

Today however, many electrical systems also have harmonic currents on their lines. Harmonics are caused by non-linear or pulsed loads (such as electronic power supplies), and their current causes the apparent power to exceed the active power by a substantial amount. In these situations, the form of power factor present is called distortion power factor. The sum of the displacement and distortion power factors is the total power factor (TPF).

For linear loads, you can make measurements to determine displacement power factor with a number of handheld instruments. These instruments can measure kilowatts (kW) and kilo-volt-amperes (kVA), and some can directly read power factor (PF). When harmonics are present, meters with true rms capability must be used to accurately account for the total cur rent (the current at the fundamental 60 Hz and the harmonic currents) to determine TPF. Also, it's advisable to read the true rms value of the voltage, since harmonic currents may cause voltage waveform distortion in some systems.

Harmonic currents in an electrical system have frequencies that are multiples of the fundamental 60 Hz current. Thus a 5th harmonic has a frequency of 300 Hz (for a 50 Hz system a 5th harmonic would be 250 Hz). When the total current, including all the harmonic currents on the line, is used in determining the apparent power (kVA) and the active power (kW), then the TPF is equal to the rms values of kW divided by kVA.

Note that harmonics do not usually show up in kW; this is the reason harmonics tend to reduce TPF. In an electrical system, the harmonic currents caused by nonlinear loads may cause TPF to be low (.60 to .70) while the DPF could be relatively high (.90 to .95). Because of the abundance of non-linear loads now being placed on lines, you should look at PF as being the total power factor.

Be careful when adding pure capacitance. In the past, when the electrical system had a low PF, the usual procedure was to add "pure" capacitance (installing capacitors without reactors, which are used to avoid tuned circuits and to reduce harmonics). That was done when all, or almost all, of the loads had a 60 Hz sinusoidal signature. Today, adding "pure" capacitance to correct a low PF situation may cause problems due to harmonics in the electrical system. Because the impedance of capacitors decreases as the system frequency increases, and as harmonic currents are multiples of the fundamental 60 Hz current, capacitors become "sinks" that attract high-frequency currents, causing possible overheating and early failure. The cure is to install filters (a combination of capacitors, reactors, and resistors with specific design criteria) that trap the harmonics. These will improve power factor the same way as a capacitor does alone but, with the use of reactors and resistors, will reduce the flow of harmonic currents in the facility's power system as well.

Adding "pure capacitance" may create other serious problems when harmonics are present. Capacitance and inductance in an electrical system can form a "tuned" circuit where the current is resonating at a specific frequency. This is the frequency at which the capacitive reactance equals the inductive reactance. If this circuit is exposed to an "exciting" harmonic (which is at, or close to, the resonating frequency and having sufficient amplitude), the current in the circuit will oscillate, causing a circulating current that could be much greater than normal. The resultant circulating current can also produce extreme voltage distortion across all circuit elements. The high current flow can blow fuses, damage components, and reflect an excessively high harmonic level back into the entire facility. Harmonic currents negatively impact motors and transformers as well because the currents can cause overheating.

When such resonance is a possibility, there are a number of system modifications that you should consider to lessen or effectively eliminate the problem. An obvious one is to shift the resonant frequency so that it does not coincide with an exciting harmonic. This can be done by changing the capacitance (by adding or removing capacitors from the capacitor bank), or by relocating the capacitor bank to change the amount of source inductance that is in parallel with the capacitance. Sometimes, if an existing capacitor bank is able to withstand the additional duty associated with serving as a harmonic filter, the capacitor bank can be modified into a filter bank by installing appropriate rated reactors and resistors. If the resonant frequency cannot be changed, filters can be added to reduce the most troublesome harmonics. However, you must make sure that the additional filters do not cause resonance at a lower frequency.

Installing adjustable capacitor banks and harmonic filters. Some facilities install capacitor banks that are designed to correct low PF at different kVA load levels to match a facility's electrical system operation. The total capacitor bank installation might be split into four steps of PF correction, with the first step turned on at 25% of plant load, the second step at 50%, and so on. Automatic controls are supplied with the adjustable bank to determine when the switching of the steps will occur. Each step must have an independent switching device.

When switching capacitors to add or reduce the units connected to a bus, it's important for you to recognize that changes in capacitance on the electrical system introduce the possibility of causing undesirable resonance. As such, this PF correction technique must be studied carefully. Such a study usually includes testing to verify the existing harmonics in the system and the existing power factor, an investigation of the electrical system to determine the ratings and characteristics of all the equipment and all the conductors, and then an engineering or computer analysis using harmonic analysis software.

To mitigate harmonics that are present in a facility's electrical system, harmonic filters can be designed in a similar fashion to that of a capacitor bank. First, you must test for harmonics in the electrical system to determine their order and magnitude. And, resonant conditions must be checked using harmonic analysis software before the filter bank is designed. Such a design usually incorporates different combinations of filters (each filter to dissipate specific harmonics at their respective frequencies) that must be placed in and out of service in the proper sequence to avoid problems. The normal sequence begins with the lower order filters first, and then adding the higher order filters. The sequence is reversed when filters are removed from service. This sequencing is necessary to prevent parallel resonant conditions (which can amplify lower frequency harmonics) that can be caused by the higher frequency filters.

If capacitors are going to be added to a system seeing known non-linear loads, an engineering or computer study should be completed prior to the application of the capacitors. At times, the application of PF capacitors can result in a tuned circuit that can cause substantial harm to an electrical system. Engineering or computer studies (the latter using harmonic software) can indicate if filter banks are necessary for avoiding such problems and ensure proper ratings of the bank. These studies consist of doing a harmonic analysis of a facility's electrical system. To conduct such an analysis, an investigation of the existing electrical system is necessary (determination and characteristics of all loads and conductors will be needed), and information must be obtained on all new loads being planned. Prior testing of a facility's electrical system via suitable instrumentation to verify existing harmonics, PF, 24 hr load profiles, etc., would be very helpful in planning low PF and harmonic remedies. You also should establish a budget for such testing.

Determining harmonic sources. Harmonics produced by scrap metal and ladle furnaces vary due to the changing arc length over the total heat period. The amount of harmonic generation is determined by the furnace type. Scrap metal furnaces predominantly generate a 3rd harmonic voltage and produce a very erratic voltage total harmonic distortion (THD) on the bus. Ladle furnaces predominantly generate 3rd and 5th harmonic voltages and produce a more consistent THD. Not only are "odd" harmonics (3rd, 5th, 7th, etc.) produced, "even" harmonics (2nd, 4th, 6th, etc.) are also present in power systems that include furnace operation because of erratic arcing behavior yields an unequal current conduction for the positive and negative half cycles. Typical upper limits for the harmonic components of the arc voltage for both scrap metal and ladle furnaces are given in the accompanying table.

A harmonic computer analysis of arc furnace operation includes a harmonic voltage source that describes the harmonics produced by the arc furnace. An arc furnace load can be represented as a harmonic voltage source in series with some lead impedance, in this case, the impedance of the secondary cables and electrodes in the arc furnace. The lead impedance is not a negligible component and should be included in the simulation models. The harmonic study should also include the operation of the motor drive systems, if they are served by the same electrical source. The harmonics spectrum produced by 6-and 12-pulse drives are related to the pulse number of the drive.

Energizing of scrap metal and ladle furnace transformers is another harmonic current producing source. The dynamic inrush current waveform associated with transformer energizing operation includes even and odd harmonics that decay with time until the transformer magnetizing current reaches a steady-state condition. Along with the fundamental current, the most predominant harmonics during transformer energization are the 2nd, 3rd, 4th, and 5th. These harmonics normally do not cause problems unless the system is sharply resonant at one of the predominant harmonic frequencies produced by the inrush current. Then, the transformer energization will excite the system, causing voltage distortion that will affect the energizing transformer's inrush current, producing more harmonic currents and causing further distortion. This interaction can produce high values of rms and peak voltages that can degrade or damage equipment and lead to premature equipment failures.

Effects of capacitors. Capacitors can withstand a reasonable amount of harmonics. However, they may detrimentally interact with harmonic producing loads in a steel plant, like arc furnaces or rolling mills. The result can be magnification of the voltage and current distortion. The increase in the distortion is usually due to a parallel resonant condition. This condition exists when the electrical source's impedance (inductive reactance) is equal to the capacitor bank's impedance (capacitive reactance) at a common frequency. This condition produces a tuned circuit, causing an extreme voltage at that frequency.

A good indication of excessive harmonics at a capacitor bank is an increase in the number of blown fuses and eventual failure of capacitors. When fuses blow in a capacitor bank, the parallel resonant frequency will shift. The system will de-tune itself, shifting the parallel resonant point to a higher frequency, perhaps resulting in a stable operating condition. When blown fuses are replaced, problems could reoccur, returning the system to the original parallel resonant frequency that caused the initial fuse operation.

A system that has a sharp parallel resonant peak at the 5th harmonic will produce approximately 360V of 5th harmonic voltage per ampere of 5th harmonic current injected into the system. Since this harmonic current resulting from the operation of the arc furnace is significant, the resonant current could amplify the voltage at the capacitor bank, leading to capacitor failures, fuse operations, or arrester operations.

Steel mills, because of arc furnace operation and rolling mill loads, are particularly plagued with harmonics and PF problems. These loads operate at PFs that may result in penalties and lower bus voltages. The nonlinear characteristics of furnace arcs and rolling mill drives can generate significant harmonic currents that are passed into a plant's electrical system and onto utility lines.

Harmonic currents are produced at scrap metal or ladle furnaces when the harmonic voltages from the arc are impressed across the electrodes, and the load current passes through the impedance of the leads and furnace transformer. These harmonic currents are injected back into the electrical system, and usually do not cause problems unless the system is sharply resonant at one of the predominant harmonic frequencies. Then, the harmonic current can excite the resonant circuit, producing high values of rms and peak voltages. This can cause equipment damage or degradation, eventually leading to equipment failure. Severe voltage distortion can disturb electronic power supplies, such as drive systems, and may interfere with control systems.

[Figure ILLUSTRATION OMITTED]

RELATED ARTICLE: TERMS TO KNOW

Capacitor. A device for storing an electric charge. It is made of tightly wrapped layers of very thin metallic plates, separated by a dielectric material. When line voltage is applied at the terminals, capacitive reactance is introduced into the circuit. A shunt capacitor produces a leading current. Capacitance (C) is measured in farads, or more usually, microfarads ImFI.

Filter. A combination of capacitors, inductors, and resistors that are rated and configured in such a way as to reduce harmonic current at a certain frequency while exhibiting minimal impedance to the fundamental 60 Hz current. A filter acts to shunt harmonic currents, dissipating them, while at the same time being able to provide reactive power at the fundamental frequency to correct low power factor.

Reactor (inductor). A coil (with or without an iron core) that stores energy when AC current flows through it and provides inductive reactance in a circuit. A shunt reactor produces a lagging current. Inductance (L) is measured in henrys, or more usually, microhenrys (mH).

RMS (or root-mean-square). The square root of the mean of the sum of the squares of instantaneous amplitudes of a wave form; the effective value of an AC current that produces heat equal to a DC current of the same value. The rms value of a cycling function, when harmonics are present, is a more accurate value of the function compared with an average value of that function.

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